ENGRAVE follow-up of a type IIb supernova spatially coincident with the sub-threshold gravitational wave trigger S250818k

TL;DR

This paper reports on the multi-wavelength follow-up of a supernova coincident with a gravitational wave candidate, confirming it as a type IIb supernova unrelated to the GW event, and highlighting contamination issues in kilonova searches.

Contribution

The study provides a detailed observational analysis of a supernova coincident with a GW candidate, demonstrating the importance of follow-up in distinguishing true counterparts from contaminants.

Findings

Spectral classification confirmed the transient as a type IIb supernova.

The supernova's shock cooling tails are significant contaminants in kilonova searches.

The follow-up campaign strengthened the understanding of EM counterparts to GW events.

Abstract

The candidate gravitational wave (GW) event S250818k was one of only three non-retracted LIGO-Virgo-KAGRA public alerts issued during the fourth observing run of the network (O4) with a binary neutron star (BNS) merger classification probability exceeding one percent. This triggered a prompt search for a potential electromagnetic (EM) counterpart in the large localisation error region (949 deg$^2$ projected in the sky at 90% credible level). The transient SN2025ulz, discovered by the Zwicky Transient Facility (ZTF) during the search, attracted a great deal of attention due to a potential spatial and temporal coincidence, and due to its initial fast decay and featureless spectrum. Here, we report on the follow up of this transient by the Electromagnetic counterparts of gravitational wave sources at the Very Large Telescope (ENGRAVE) Collaboration. We conducted an extensive multi-wavelength observational campaign, which led to the spectral classification of the transient as a type IIb supernova (SN), indicating that it is unrelated to the candidate GW event. In this article, we describe our observing strategies, data reduction, and interpretation. All of our results confirm and strengthen our classification of the source, and also show that shock cooling tails associated with type IIb SNe are one of the most prominent contaminants in kilonova searches.